AN ABSTRACT OF THE THESIS OF
Timothy Robert Baumgartner
for the
Master of Science
(Degree)
(Name)
in
Title:
Oceanography
(Major)
presented on
December 20, 1973
(Date)
TECTONIC EVOLUTION OF THE WALVIS RIDGE AND
WEST AFRICAN MARGIN, SOUTH
Abstract approved:
TLANTIC OCEAN
Redacted for
Redacted
forprivacy
privacy
Tjeerd H. van Andel
The opening of the South Atlantic between 140 and 90 m. y. B. P.
occurred about two poles of rotation. The initial pole of rotation was
maintained until Africa and South America were completely separated.
The subsequent removal of restraints imposed by the pre-existing
structure of Africa and South America on the early spreading
direction permitted a northward migration of the pole of rotation
and concomitant reorientation of spreading direction,
The NNE trending Cameroon-Gabon and southern Angola coast-
lines which offset the generally SSE trend of the west African margin
between 5°N and the Walvis Ridge are proposed as initial transform
offsets of the South Atlantic proto-rift.
Location of these initial
offsets was controlled by lineations of the Precambrian/Early
Paleozoic belts of thermotéctonic activity which occur between older,
stable cratonic nuclei of Gondwana. Shorter offsets of the continental
margin south of the Walvis Ridge and the two major offsets of the
west African coastline north of the Walvis Ridge define a pole of
rotation at 5°N, 26°w for the initial South Atlantic opening.
A set of magnetic anomaly lineations near the continental
margins of Angola and southwest Africa is described and named the
Benguela sequence.
These anomalies were formed during the initial
phase of spreading and are displaced right-laterally almost 1000
kilometers across an extension of the continental offset along the
southern Angola margin. This offset is named the Benguela Fracture
Zone.
The Benguela anomalies are correlated with anomalies of the
Lynch sequence in the western North Atlantic. The change in
direction between the two pre-Cenozoic phases of South Atlantic
spreading is dated at roughly 120 m. y. B. P. based upon an extrapolation of the ages of anomalies in the Lynch sequence to the time of
reorientation in the South Atlantic. Formation of the South Atlantic
quiet zones occurred by sea-floor spreading about a pole of rotation
at 21. 5°N, 14. 0°W during the second phase of opening.
The present structure of the Walvis Ridge is controlled by
nearly orthogonal NE and NW trending faults shown by seismic
reflection profiling. The NE trending set of faults approximate small
circles of the pole of rotation at 21. 5°N, 14. 0°W and appear to offset
the Walvis Ridge topography in a right-lateral sense. Reorientation
of spreading would have produced extension across the Benguela
Fracture Zone; development of short offset spreading centers along
the Benguela Fracture Zone during this reorientation is proposed to
explain the right-lateral offsets of the Walvis Ridge topography.
Lack of geophysical information on the lower crustal structure
prevents a direct explanation of the present elevation of the Walvis
Ridge. However the Walvis Ridge is probably underlain by a low
density root produced by alteration of the lower crust and upper
mantle materials beneath the Benguela Fracture Zone which began
during the spreading reorientation. Asynimetric spreading over the
Walvis Ridge may have permitted the zone of crustal accretion to
remain near the older Benguela Fracture Zone long enough to allow
the creation of an anomalously broad low-density root which is
responsible for the uplift of the Walvis Ridge.
Tectonic Evolution of the Walvis Ridge and
West African Margin, South Atlantic Ocean
by
Timothy Robert Baumgartner
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Master of Science
June 1974
APPROVED:
Redacted for privacy
Professor in Oceanography
in charge of major
Redacted for privacy
Dean of the Sc\iool of Oceanograhy
Redacted for privacy
Dean of the Graduate School
Date Thesis is presented
Typed by Linda Wille for
December 20, 1973
Timothy Robert Baumgartner
ACKNOWLEDGEMENTS
Professional gratitude is hardly enough for the faith and
encouragement extended to me by Dr. Jerry van Andel, my major
professor. He allowed me the freedom to pursue this study through
periods of unsettling personal vicissitude with nothing but positive
support throughout. He initially suggested a study of the Walvis
Ridge and provided the CIRCE 8 data from Scripps Institution of
Oceanography.
Ken Keeling, Mike Gemperle and Nick Pisias provided me with
advice and assistance in the computer processing of magnetic data.
I am grateful for their patience. Ken Keeling also adapted the
"Hypermap" program of R. L, Parker for use with the OS-3 system.
Many of the ideas in this thesis germinated during discussions with
my fellow students. Talks with David Rea and Bruce Malfait were
particularly helpful in clarifying some of my more muddled thoughts.
Drs. Richard Blakely, Ted Moore and Julius Dasch read the thesis
and gave me valuable suggestions.
Dr. Eric Simpson provided bathymetric and magnetic profiles
coUected and compiled by the University of Cape Town, South Africa;
Dr. Hartley Hoskins of Woods Hole Oceanographic Institute provided
the seismic reflection, bathyrnetry and magnetic data collected during
Leg 4 of the R. V. Chain 99 cruise; and Dr. Water Pitman of
Lamont-Doherty Geological Observatory provided magnetic data taken
on R.V. Conrad 1313 cruise. Without their generosity this thesis
would have been much shorter.
I thank Kathryn Torvik, who drafted the figures and Linda Wille,
who typed final drafts of the thesis.
Barbara Gwinner deserves special personal thanks for the warm
companionship and uplift she gave at times when this work seemed to
hold little reward. Finally, I thank my parents whose support and
encouragement have provided the foundation of my education.
Funding for this study was provided by the National Science
Foundation grant GA3 1478 and Office of Naval Research grant
N000 14-67-A-- 0369-0007.
study history means submitting to chaos and
nevertheless retaining faith in order and meaning."
"TO
Father Jacobus to Joseph Knecht
in The Glass Bead Game
by Herman Hesse
TABLE OF CONTENTS
INTRODUCTiON
1
DATA SOURCES AND CONTROL
4
STRUCTURAL CONTROL OF THE WEST AFRICAN MARGIN
5
PRE-CENOZOIC MAGNETICS AND SPREADING HISTORY
23
RELIEF AND STRUCTURE OF THE WALVIS RIDGE
46
TECTONIC EVOLUTION OF THE WALVIS RIDGE
56
BIBLIOGRAPHY
73
LIST OF FIGURES
Figure
Page
1
Structural orogenic units of Africa
2
Location of Libreville and Benguela lines
11
3
Location of Luderitz and Cape Town lines
16
4
Mercator projection of Africa shifted to
pole at 5. 0°N, 26, 0°W
21
5
Correlation of Benguela magnetic anomalies
24
6
Map of Benguela anomalies
26
7
Correlation of anomalies within the Lynch and
Keathley sequences to anomalies of the
Benguela sequence
31
8
9
10
11
12
Location of Angola Basin and Cape Basin
magnetic quiet zone boundaries
14
15
36
120 and 90 m. y. B. P. reconstructions of
the South Atlantic
41
Basement structure of the Walvis Ridge east
of 4°E superimposed on bathymetry
47
Line drawings of seismic reflection profiles
across Walvis Ridge
50
Paleotectonic interpretation of early South
Atlantic opening at approximately 130 m. y.
B.P.
13
6
57
Opening of the South Atlantic at approximately
90m.y. B.P.
60
Postulated reorientation of short spreading
centers within the Benguela Fracture Zone
64
Proposed model for tectonic evolution of the
Walvis Ridge
67
LIST OF TABLES
Table
Page
Rotations used for Figures 9 and 13
43
INTRODUCTION
The Walvis Ridge is a sinuous elevation of the ocean floor
linking the continental margin of South-West Africa with the flank of
the mid-ocean ridge of the South Atlantic, Except for one earthquake
(Stover, 1968) just south of the Walvis Ridge, it is aseismic, a relic
of crustal processes operating as Africa and South America separated.
This paper traces the evolution of the Walvis Ridge within the framework of the Pre- Cenozoic history of the South Atlantic,
The concept of seafloor spreading (Hess, 1962; Dietz, 1961) has
been broadened with the formulation of plate tectonics by McKenzie
and Parker (1967), Morgan (1968), and LePichon (1968). Most of the
earth's seismicity, tectonism and orogenesis can be explained by
movement along the boundaries of a number of rigid plates which are
in motion with respect to each other. Symmetric magnetic anomaly
patterns about mid-ocean ridges resulted from reversals of the
earth's magnetic field during accretion at plate boundaries (Vine and
Matthews, 1963; Vine, 1966), Using these anomaly lineations as
crustal isochrons and the strikes of transform faults (Wilson, 1965)
as flow lines, past configurations of continents and the resultant
shapes of ocean basins can be reconstructed (Pitman and Taiwani,
1972; McKenzie and Sclater, 1971), The relativeplate motions
required for these reconstructions can be described by finite poles
and angles of rotation,
a
The spreading history of the South Atlantic from latest
Cretaceous to the present is relatively well known from studies of the
magnetic anomalies and from Deep Sea Drilling (Dickson and others,
1968; Heirtzler and others, 1968; Maxwell and others, 1970).
Mascle and Phillips (1972a) have related symmetric magnetic quiet
zones in the South Atlantic to seafloor spreading during a period of
normal polarity in the late Cretaceous, LePichon (1968) pointed out
that the opening of the South Atlantic must have occurred about at
least two successive poles, Assuming the present pole of rotation
(58°N, 37°W) to be valid for the entire Cenozoic, an average preCenozoic pole at 35°N, 21°W is needed to satisfy the reconstruction
of Gondwana by Bullard and others (1965). Small circles about this
early pole approximately parallel the northern scarps of the Walvis
Ridge, Rio Grande Rise, and Falkland Plateau, LePichon and Hayes
(1971) proposed a model for the development of continental margin
offsets and associated ridges and fracture zones in the South Atlantic,
They believe that the evolution of the entire South Atlantic between
140 and 80 n-i, y. ago can be described by a pole of rotation at 21, 5°N,
14. 0°W, derived from equatorial fracture ridges (St, Paul, Romanche.
and Chain), More recently Francheteau and LePichon (1972) fitted
structural trends along the South American and African margins
(considered by them as original transform directions) to flow lines
of this early pole of opening. Although small circles about this pole
3
do agree with the scarps along the northern boundaries of the Walvis
Ridge, Rio Grande Rise and Falkiand Plateau, they do not coincide
with the major continental offsets of the African margin along the
Cameroon-Gabon and Angola-South West Africa coasts.
LePichon (1968) notes that, given an underlying state of stress,
a thick lithosphere should break along zones of internal weakness and
that patterns of oceanic opening may be determined by pre-existing
structure, It is likely that the initial opening of the South Atlantic
was controlled by major structural boundaries within the Gondwanan
lithosphere. These boundaries were the loci of episodic orogeny and
tectonic rejuvenation throughout the Precambrian, and their effect on
the later tectonic evolution of Africa is well documented (Clifford,
1968, 1970), This paper examines the history of the South Atlantic
in an attempt to relate the effects superimposed by the pre-existing
basement of Gondwana to the creation of the adjacent seafloor and
concomitant evolution of the Walvis Ridge. Magnetic anomalies in
the eastern Angola and Cape basins and on the continental rise off
Africa, described in this paper, yield a seafloor spreading pattern
which is consistent with the directions of major internal weaknesses
in Africa. The structure of the Walvis Ridge is interpreted with the
assumption that rifting took place along these continental zones of
weakness.
DATA SOURCES AND CONTROL
The data for this study were collected on expeditions from
various institutions and include underway measurements of
bathymetry, magnetics and seismic reflection. Magnetic data were
recorded on Legs 8 and 9 of the CIRCE Expedition from Scripps
Institution of Oceanography, Leg 4 of cruise 99 of R. V. Chain from
Woods Hole Oceanographic Institute, the R,V, Conrad cruise 1313 of
Lamont-Doherty Geological Observatory, and traverses 2, 71, 204,
and 205 of the Department of Geology of the University of Cape Town,
South Africa. Aeromagnetic data from PROJECT MAGNET flights
(Mascie and Phillips, 1972a) of the U.S. Naval Oceanographic Office
were also utilized. Continuous depth and seismic reflection profiles
used in the study are from the CIRCE 8;and Chain 990/4 cruises.
All magnetic measurements were made with a proton precession,
Varian magnetometer. Magnetic anomaly profiles were obtained by
elimination of the regional field using the IGRF coefficients (Cain
and others, 1968). Seismic reflection profiles were made on CIRCE
8 with a 10 cubic-inch airgun and on Chain 99/4 by firing a 40 cubicinch airgun and 100, 000 joule sparker synchronously.
Prithary
navigation during all cruises except those of the University of Cape
Town was achieved by satellite navigation. The South African
traverses utilized celestial navigation.
5
STRUCTURAL CONTROL OF THE WEST AFRICAN MARGIN
During the breakup and dispersal of Gondwana, Africa acted as
a single crustal block. However, Precambrian and early Paleozoic
belts of orogenesis and syntectonic radiometric rejuvenation indicate
a long preceding period of mobility throughout mid-continental regions
of the African basement, Subsequently, since the early Paleozoic,
deformation of rocks has occurred only along the continental penphery, in the Cape Fold Belt, the Mauritanides and the Atlas
Mountains.
Within the African interior, great sedimentary basins
began forming during the time spanning the Precambrian/Paleozoic
boundary. The development of these sedimentary basins persisted
throughout the Paleozoic and Mesozoic into the Tertiary, masking
large regions of the Precambrian basement with sediments but
leaving the outer rims of the interior basins exposed. In recent
years, extensive work in radiometric dating of African rocks exposed
above the platform cover has provided a chronologic framework for
the evolution of African Precambrian and early Paleozoic structural
and stratigraphic provinces. Radi,ometric dates of the Precambrian
and early Cambrian basement fall into four groups which broadly
reflect times of orogenesis and thermal overprinting. These are
>2100 m. y., 2050-1600 m. y. , 1050-850 m. y. and 700-400 m. y.
(Clifford, 1970), This progression of activity resulted in the
FIGURE 1: Structural-orogenic units of Africa (after Clifford, 1970).
WEST
AFIAN'
CRTON
k4' /',
/7
Il\
1%
CO
REGIONS AFFECTED BY
II
PAN-AFRICAN OROGENESIS
1./i
(00-400
STABLE
Lfl
1100
STABLE
tn 1600
MAJOR
M.Y.
M,Y.
B.R
SINCE
M.Y,
b.I1'
SINCE
B.R
FAULTS
\
A
,:<_- (I
I... .......
,'z,
RI
( /
TOi
/
successive blocking-out of three major, stable cratonic nuclei
(Figure 1). The regions of the West African, Congo and Kalahari
nuclei were stabilized by approximately 1600 m. y. ago, although
orogenesis after 1100 m0 y. ago increased the area of the Kalahari
craton and divided the Congo craton. Structural differentiation of the
African basement culminated between 700-400 m. y. ago in a wide-
spread thermotectonic episode called the Pan-African event by
Kennedy (1965).
Orogeny and rejuvenation by thermal overprinting
of the Pan-African event affected almost the entire continent outside
the stable cratonic nuclei, By the end of this event, the stable cratons
were surrounded by long, sinuous belts of rocks with a mega-tectonic
grain produced by folding of geosynclinal sediments, widespread
metamorphism of older schists and gneisses, and syntectonic
emplacement of granites (Clifford, 1968). These tectonic and
metamorphic lineations are generally parallel to the strike of the
Pan-African
belts0
Clifford (1968) notes that all major fault systems of the Phanerozoic except the Western Rift Valley, are located within zones of
Pan-African rocks. Wilson1s (1965) concept of transform faulting
predicts that a proto-rift through continental crust would prefer preexisting lines of weakness. It seems reasonable that the structural
heterogeneity developed throughout the Precambrian should have
determined the initial rifting configuration between Africa and
South America.
LePichon and Hayes (1971) predict the existence of fracture
ridges within rifted continental margins which are inferred counter-
parts of fracture zones along mid-ocean ridges. Francheteau and
LePichon (1972) attempted to locate such fracture ridges by the
association of structural and bathymetric trends with offsets of the
South American and African margins. Their search concentrated on
trends that agree with the pole of rotation for the early South Atlantic
obtained by LePichon and Hayes (1971). However, a gravity survey
of the Angola margin (Rabinowitz, 1972) has cast doubt on the fracture
ridges postulated there by Francheteau and LePichon. Rabinowitz
points out that geophysical data from many inactive continental
margins do confirm or support the presence of basement ridges
(Drake and others, 1959; Rabinowitz and Talwani, 1969; Ewing and
others, 1963; Emery and others, 1970), and Burk (1968) suggests
that these ridges mark the initiation of seafloor spreading. However,
the extent to which these ridges represent transform shearing, block
faulting of the margin or any other process which might be associated
with rifting, is still not well understood.
Four major trends along the continental margin between
Cameroon and South Africa agree with onshore tectonic and meta-
morphic lineations within the basement regions affected by the PanAfrican rejuvenation. I have called these the Libreville, Benguela,
10
Luderitz and Cape Town lines (Figures 2 and 3). Both the Libreville
and Benguela lines are directly juxtaposed to Pan-African belts, and
follow along lobes of the Congo craton. The Luderitz and Cape Town
lines lie along bathymetric trends extending off the continental shelf
to the continental rise,
Within Cameroon, south and east of the Cameroon volcanics and
north of the Congo craton, tectonic and metamorphic lineations trend
between N10°E and N20°E near the coastline (Figure 2). Here the
migmatiteJ.ineations and folded belts are more ancient structures that
were rejuvenated during the Pan-African event, Away from the
coast, these NNE lineations bend to the east around the Congo craton
and branch south, The Cameroon volcanics near the coast are
emplaced within the Pan-African zone along a boundary zone which
marks a slight convergence of the migmatite lineations. To the east,
these volcanics lie along a fault system which itself is apparently
controlled by the trend of the Precambrian migmatites and syntectonic
granites, The base of the continental slope (2000 meter contour) from
3°N to l°S, along the Cameroon and Gabon coasts is parallel to
onshore Pan-African migmatite lineations and fold axes which bend
around the Congo craton. Although the coastline cuts through
cratonic rocks from Bata to Point Gentil, the spatial relation of the
Pan-African zone to the Congo craton suggests that the border of the
Pan-African rocks extends under the continental terrace to the south-
FIGURE 2: Location of Libreville and Benguela lines along west
African margin. Tectonic map simplified from
UNESCO-ASGA (1969) 1:500, 000 International Tectonic
Map of Africa. Bathymetry from Uchupi (1971).
12
10'
IS.
MAP
LOCATION
l0
CHAD
..
NIGERIA (
..REPOF
\
:j.:cAMEROON
CENTRAL
AFRICA..
FP(.
BATAAiO ...........
EEE('-
-
5'
REVILLE
00
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/
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\
i REP OF
JGABoN
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\
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,CONGO,.:
GENTIL
REP OF
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ii)
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CABINDA
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''t
100
NOvAS
0°
REDONDOJ.
J:ANGOLA
Jiji
.
(I
________
_____
-_____
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1.t*4
.,.-.
.j.;
'c'
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200
AFRICA
5'
(( ci
i4j
,iI¼ J
\
-,
I
I
\
LEGEND
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\ \\ \
\ \\ \
LlTHo-oROENIc
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10'
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\\\\
\
I
I
PLATFORM COVER
'
AND BASEMENT
rON
j
I
REJUVENATION
\(
\
COASTAL BASINS
PAN-AFRICAN OROGENIES
(WEST CONGO AND DAMARA)
'?
\
UNITS
POST PALEOZOIC AND
KAROO PLATFORM COVER,
LOWER CAMBRIAN AND
UPPER PRECAMBRIAN
I
\
200
100
1 1'
I
I
\
BAY:2...
',.jA_____
)
.'.-
WALVIS
'I
f:::.:::
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CRATONIC NUCLEII
STABLE SINCE
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IS'
TECTONIC AND METAMORPHIC
STRUCTURES
\\
TRENDS WITHIN
/-
1
I
FOLDED ZONES
'\
I
FAULTS
SYNTECTONIC
GRANITES
20'
'
-i
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AMA RIDE
.j'
250
h
MIGMATITES
]
eI;öi:!;ii
VOLCANIC
I'1
ACTIVITY
EXTRUSIVE CAMEROON
AND KAROO VOLCANJCS
BASIC INTRUSIVES
13
west. Recent work by Pautot arid others (1973) has shown that the
salt diapir limit of the Gabon Basin follows the 2000 meter contour
between 1 °N and 3°S.
They suggest a relationship to a fracture zone
trend that is postulated by Burke (1969) which reaches the continent
at 2°N near Bata with a strike of N45°E. It is reasonable to assume
that the northwestern limit of salt deposition was controlled by a
fracture zone, but the trend and location are more probably those
of the Libreville line (Figure 2). Although this fracture zone is
thought to have limited the extent of evaporite sedimentation, neither
Pautot and others (1973) nor Leyden and others (1972) have found
evidence for a NNE trending basement ridge in seismic reflection
profiles across the slope break. Possibly, a once existing basement
ridge may have subsided and gradually disappeared beneath the
sediments of the continental slope and rise,, Hurley and Rand (1969)
show that a small cratonic nucleus in northwestern Brazil has broken
from the Congo craton, However, between this small cratonic nucleus
and the Brazil margin, there is a northeast-southwest trending
province dated 400-800 in. y. B. P., which is the counterpart of the
Pan-African rejuvenation. This evidence points to an approximate
strike of N10°E for the direction of shearing along the Libreville line.
This line follows the 2000 meter contour until it breaks to the southeast off Gabon. The Cameroon volcanics south of 7°N which describe
a slightly different direction than the Libreville line, lie along a
14
boundary within the Pan-African belt itself where the Pan-African
metamorphic lineations converge. An extension of the Libreville
line landward would meet this boundary at a low angle. Since the
Cameroon volcanoes probably did not develop until the Neogene
(Burke and others, 1971), they may represent a reactivation of
stresses along slightly different Pan-African directions than those
from which the initial South Atlantic rifting developed.
The coastline of Angola, from Nova Redondo to Mossamedes,
skirts the edge of the Congo craton (Figure 2). The zone of PanAfrican rejuvenation lies within a narrow band along the edge of the
craton and follows earlier Precambrian trends of syntectonic granite
emplacement within the Congo craton of southern Angola. The
opposing coastline of Brazil (Bullard and others, 1965) along the
Sao Paulo embayment lies within the much wider Paraiba metamorphic belt which is also dated as 5 00-700 m. y. B, P. (Cordani and
others, 1968) and is dominated by structural lineations which were
parallel to the Pan-African trends in Angola when the two continents
were joined. Off Angola a double band of isostatic gravity highs
extends southwestward from 9°S to the Walvis Ridge (Rabinowitz,
Figure 5, 1972). The axial trend of these anomalies is N20°E,
although the seaward gravity high is offset by minor north- south
trends. The western gravity high approximates the upper continental
slope which has an average trend of N19°E. Between 10°S and 15°S,
15
the trend of the Pan-African zone also averages N20°E. From
comparison with a study of the Norwegian margin by Taiwani and
Eldholm (1972) Rabinowitz concludes that the seaward gravity high:
marks the oceanic-continental boundary and that the estern gravity
belt ma be a hinge line for subsidence within the Precambrian rocks.
Seismic reflection data over the Walvis Ridge-continental slope join,
discussed later, record an uplifted basement block which is
associated with the seaward free-air gravity anomaly (Rabinowitz,
Figure 4, 1972). Basement fault scarps parallel to this block cross
the more EW topographic trend of the Walvis Ridge here. Strikes of
these faults are parallel to the axes of the positive gravity anomalies
described by Rabinowitz. The Benguela line in Figure 2 is parallel
to the axial trend of the double band gravity highs, to the Pan-
African trend, and the general trend of the continental slope. The
strike of this line represents the logical direction for transform
shearing along the southern Angola margin. It is again noteworthy
that the belt of salt diapirism south of 11 °S defined by Pautot and
others (1973) is nearly parallel to the Benguela line, However, its
seaward limit appears to extend slightly beyond the Benguela line
to the west.
The Libreville and Benguela lines occur where Pan-African
belts abruptly change directions conforming to the boundaries of
cratonic nuclei, This is not true for the Luderitz and Cape Town
16
FIGURE 3: Location of Luderitz and Cape Town lines along the
southwest African margin. Bathymetry after Simpson
and Forder (1967).
17
20°E
IOE
25°S
-I---
25S
)
(v
1
1/
\
'V
\
Agulnas
Bank
SeamounN,/
Eric a
Seamount
hmitt Ott
Fanza ri ni
Sea mo7
Seamount
lines (Figure 3), Although the Pan-African Damara belt north of
Luderitz curves southwestward following the Kalahari craton, this
segment of the coastline has no major offsets like those bordering the
Congo craton. Rather, the Luderitz and Cape Town lines are
recognized by regional offsets of the continental margin bathymetry,
which are analogous to more local offsets of the shelf break between
Walvis Bay and Cape Town, Although it is not displayed well in
Figure 3, van Andel and Calvert (1971) and Dingle (1973) have noted
a regular scalloped morphology of the continental shelf and suggested
both erosional processes and tectonic control for its origin, Near
27. 5°S, the continental shelf break along the 200 meter contour turns
abruptly into a southwestern trend which continues to the 3000 meter
contour, Paralleling this offset of the shelf break to the southwest,
a fault scarp has been mapped by Dingle (1973, Figure 7) between
the 200 meter and 500 meter contours. At 33, 5°S a narrow topographic high extends from the continental shelf to the 4500 meter
contour (Figure 3). This lineation borders the NE-SW trending Cape
Canyon incised across the continental shelf (Dingle, 1973, Figure 7).
The residual magnetic anomaly pattern off southwestern Africa
(Simpson and Du Plessis, 1968, Figure 4) exhibits a general northwesterly trend, Inner and outer belts of high amplitude (up to 500
gammas) positive anomalies parallel the continental shelf break.
A northeasterly cross trend is imposed on the primary direction of
19
these belts between Walvis Bay and Luderitz, which Simpson and
Du Plessis suggest is due to offshore extensions of Damaran rocks
with high magnetic susceptibilities. The NE trending shelf break
segments mapped by van Andel and Calvert (1971) (included in Figure
6 for reference) fall along the three prominent positive magnetic
cross trends and appear to reflect a magnetic edge effect across
faults which must control the morphology of the continental shelf and
slope there, The bathymetric lineations used as control for the
Luderitz and Cape Town lines in Figure 3 occur along offsets of the
outer positive magnetic belt. This evidence indicates that the
structure of the African continental margin from the Walvis Ridge to
Cape Town is controlled by NE-SW offsets of the basement even
though there are no onshore tectonic or metamorphic lineations of
this orientation within the Pan-African basement, The free-air
gravity map of Simpson and Du Plessis (1968, Figure 5) shows a
positive belt of gravity highs which follow the upper continental slope
between Walvis Bay and Cape Town. Although the axis of this belt is
offset right-laterally as is the shelf break, there are only very weak
northeasterly trends of the kind that might be associated with
fracture ridges along transform directions. Seismic reflection
(Simpson, 1971) and sonobuoy refraction data (Bryan and Simpson,
1971) between Luderitz and Cape Town yield no evidence for base-
ment ridges, although a discontinuity in the basement below the Cape
20
Canyon suggests faulting. Considering Rabinowitz's conclusions for
the structure of the Angola margin, it is likely that the positive
gravity belt along the slope marks the boundary between continental
and oceanic crust. Nevertheless, lateral offsets of the continental
crust, represented by the Luderitz and Cape Town lines, are strongly
indicated, and these features are considered to be the directions of
initial transform shear south of the Walvis Ridge.
The Libreville, Benguela, Luderitz and Cape Town lines fall
along small circles of a pole located at 5°N, 26°W. A Mercator
projection of Africa (Figure 4) with its pole shifted to these coordin-
ates, demonstrates the parallelism of these four lines and suggests
a mutual origin related to the initial rotation between Africa and South
America. If this is true, these lines are parallel to, if not actually
coincident with, early Cretaceous fracture zones created along the
Atlantic proto-rift. These fractures appear to have had a controlling
influence on the present morphology and history of sedimentation
along the continental margin from the Gulf of Guinea to the tip of
South Africa.
21
FIGURE 4: Libreville, Benguela, Luderitz and Cape Town lines
along the west African margin: Mercator projection
shifted to pole at 5°N, 26°W. The fit of these postulated
lines of shear may be compared to small circles about
this pole since all parallels of the pole are horizontal
in this projection. Dashed lines are parallels and
meridians about present geographic north pole.
Ce
i
----
3OS-
---
LIBREVILLE LINE
'
BNGUELA LINE
.
:.. .. .
.
..........
LUDERITZ LINE
I
cA:Tor
TTTT
N)
N)
23
PRE- CENOZOIC MAGNETICS AND SPREADING HISTORY
Two sets of magnetic lineations are present within an area
bounded by 5°S, 40°S, 0°E and the African margin. The landward
set of anomalies, called here the Benguela sequence, is thought to
reflect spreading along a direction parallel to the Benguela line. The
seaward set includes the magnetic quiet zone (van Andel and Moore,
1970; Mascie and Phillips, 1972a) and anomalies to the west within
the Heirtzler sequence (Heirtzler and others, 1968).
The Benguela sequence has been constructed from correlations
between magnetic profiles taken on Chain 99/4, CIRCE 8, CIRCE 9,
Conrad 1313, and South Africa cruises 2, 71, 204 and 205 north and
south of the Walvis Ridge. Profiles over the Walvis Ridge show that
the magnetic signature is disturbed by basement structure and
morphology, For simplicity, these profiles are omitted from the
presentation except for one Walvis Ridge crossing on Chain 99/4.
Most of these profiles are oriented northeast-southwest, making it
possible to observe magnetic lineations nearly perpendicular to the
strike of the Benguela offset.
The more east-west oriented profiles
(CIRCE 9, South Africa 204, 205 and the north-south segment of
Chain 99/4) were projected to an azimuth of 035°.
After projection of the profiles, visual comparisons of the
relative number, sequence, and amplitudes of the maxima and
24
FIGURE 5: Correlation of Benguela anomalies along ship
tracks shown in inset. Correlations are made between
multiple peak anomaly groups rather than between
individual anomalies. The anomaly groups are identified
by Roman numerals which correspond to the linear bands
on Figure 6, Anomaly profiles of CIRCE 9, the northsouth segment of Chain 99/4, and South Africa 204 and
205 are projected to 035° to improve identification.
Fracture zones shown as continuous lines to aid
comparison with Figure 6. Stippled tone indicates
anomalies not related to seafloor spreading. Scarps
along the continental shelf interpreted as faults are
noted by F on the magnetic profiles. The magnetic
profiles are arranged in relative geographic position
from north to south; their relative east-west positions
are shifted slightly to permit easier visual tracking of
anomalies. Parallels (below baselines) and meridians
(above baselines) are shown by tick marks every four
degrees. SM indicates seamount anomaly.
25
I
40Q
CIRCE 9
I
200L_200
CIRCE 8
A
C0NR
1313
CHAIN
99/4
5
10
15
I0
15
A1LG0LA
SOUTH-
20
25
WE ST
RICA
IE
-;
IL
.i
i\
i\
c7
I
/'I
A4
L
26
FIGURE 6: Map of Benguela anomaly correlations in Figure 5
plotted along ship tracks which are shown as light
stippled 1ines Fracture zones shown as heavy dashed
lines, Scarps marking continental shelf breaks mapped
by van Andel and Calvert (1971) and Simpson and
Du Plessis (1968) between 20°S and 27°S drawn with
narrow dot-dash lines, Magnetic quiet zones, shown
as grey stippled pattern, and anomaly 33 taken from
Figure 8, 2000 and 4000 meter contours from Uchupi
(1971),
27
5°
00
50
0°
5°-
5°
\\
oc
/
/
N
I
/
A
/
5C
200
\
V/S
4Y
25°
N'
300
\
<c
350
minima were made. The correlations are shown in Figure 5 as bands
of multiple peak anomaly belts and identified with Roman numberals
which begin with the oldest, Figure 6 is a map of the anomaly belts
plotted along the ship tracks,
The best overall correlation of the anomalies is between the
longer Chain 99/4 and CIRCE 8 profiles north of the Walvis Ridge.
These anomalies include those recognized by Von Herzen and others
(19-72) from CIRCE 8, CIRCE 9, Chain 99/4 and Chain 99/3 in a small
area north of l2°5. A right-lateral magnetic offset of nearly 1000
kilometers across the Benguela line yields further evidence for a
major fracture zone along the Angola margin here (Figure 6), Below
the Benguela fracture zone the anomaly correlations are complicated
by numerous crossings of three closely spaced fracture zones
(Figure 6). Only profile South Africa 2, south of the Walvis Ridge,
does not cross any fracture zones. In this region van Andel and
Calvert (1971) mapped three NE-SW offsets of the NW-SE scarp which
marks the shelf break, These offsets of the shelf edge appear to
extend seaward and displace the magnetic anomalies. South Africa
204 and 71 and Conrad 1313 show large amplitude negative anomalies
where these tracks cross the NE-SW striking offsets of the shelf.
Where the seaward extensions of the fracture zones cross magnetic
profiles South Africa 71 and Conrad 1313 they show significant
changes in magnetic signature with subdued and slightly negative
anomalies. The South Africa 71 crossing of the seaward fracture
zone is wide and more pronounced, probably because the track line
crosses it at a low angle. The single fracture zone mapped above the
Benguela line, north of the Waivis Ridge, would meet the shoreline
near 6°S if extended to the northeast. Onshore, the fold trends
within the West Congo orogenic belt of Pan-African age make an
almost 900 change in strike (Figure 2) east of Cabinda,
This fracture
zone appears to be an initial offset of the continental margin that may
have been directly influenced by the structural trends of the PanAfrican basement as along the Libreville and Benguela lines.
Rabinowitz (1972) notes that the magnetic offset mapped initially by
Von Herzen and others (1972) lies along a wide gravity high which
may suggest an underlying basement ridge associated with shearing
along the fracture zone,
Magnetic anomalies related to the structure of the continental
margin, indicated by the stippled tone in Figure 5, show no coherent
pattern of lineation. Baumgartner and van Andel (1971) and Von
Herzen and others (1972) have related the shoreward anomalies at
9°S to northwesterly basement ridges extending from the continent.
South of 20° S, there are strong negative anomalies over the SE-NW
trending shelf scarps between the right-laterally offset NE-SW s carps,
Thus the SE-NW striking scarps are thought to be hinge faults along
which subsidence of the continental margin occurred between the
30
fracture zone offsets. These scarps are noted in Figure 6 also.
The strike of the Benguela anomalies is best determined by the
lineations north of the Walvis Ridge on the two long tracklines which
do not cross fracture zones. Here the strike is generally N45°W
except for anomaly IX, which strikes N25°W.
Vogt and Johnson (1971) have established an anomaly sequence
(Lynch sequence) in the western North Atlantic between the Keathley
(Vogt and others, 1971) and Heirtzler magnetic sequences. The
boundary between the Keathley and the Lynch sequences lies along
the 135 m. y. B. P. isochron of Pitman and Taiwani (1972). This
isochron is based on extrapolated spreading rates since no JOIDES
holes penetrate to basement within this area. Comparisons of
anomalies from the Keathley, Lynch and Benguela sequences
are shown in Figure 7. No dates are available for the Benguela
anomalies, but since B-I must be contemporaneous with the initiation
of spreading in the South Atlantic, a rough upper age limit can be
assigned. Although postulated dates for this event vary from 200 m. y.
B. P. to 90 m. y. B, P., the most reasonable estimates fall around
the Jurassic/Cretaceous boundary (140 to 130 m, y. B, P.). Dietz and
Holden (1972) assume 135 m, y. B, F,, which agrees with radiometric
dating of the Kaoko basalts (136 to 114 m, y. B, P.) of Southwest
Africa (Siedner and Miller, 1968) and Serra Geral lavas of Brazil of
the same age (Amaral and others, 1966). Deep Sea Drilling data in
31
FIGURE 7: Correlation of magnetic anomalies in the western North
Atlantic (Lynch and Keathley sequences) from Vogt and
Johnson (1971) and in the eastern South Atlantic (Benguela
sequence). Note differences in scales for each region.
Time ticks shown for Keathley and Lynch sequences based
on 135 m. y. isochron in western North Atlantic and onset
of Late Cretaceous magnetic quiet period.
32
E4OO
CHAIN99/4
VV\j
IN
-2DO
0
go
0
SOUTH AFRICA 71
CONRAD 1313
_3 3
the South Atlantic yield an extrapolated date for opening near 130 m, y.
B. P. (Maxwell and others, 1970). LePichon and Hayes (1971) chose
140 m. y. B. P. as the initiation of spreading. Therefore an age
between 140 and 130 m, y. B. P. has been assumed for B-I. Thus, it
is reasonable to assume that the Lynch and Benguela sequences were
initiated at the same time and correlation of their anomalies should
be possible.
The Lynch sequence lies approximately 14° to 20° north of the
Cretaceous magnetic equator (determined from a continental North
America paleomagnetic pole at 70°N, 170°W; Larochelle, 1969)
while the Mesozoic paleolatitudes of Angola and South West Africa,
(McElhinny and others, 1968) are between 20° and 30°S. Accordingly,
the larger amplitudes of the Benguela anomalies suggest a higher
paleolatitude than the Lynch anomalies, Because the profiles are
reversed and from different hemispheres, the shapes of anomalies
should be asymmetric with respect to one another. This asymmetry
is pronounced in the anomaly band IV between the CIRCE 8, Conrad
1313 profiles and the C, F profiles in the North Atlantic.
Both sets
of anomalies have approximately symmet?ic orientations to magnetic
north (Lynch, N30°E; Benguela, N45°W),
The end of the Lynch anomaly sequence has been placed at 90
m. y. by Vogt and Johnson (1971). Since they note, however, that
the Lynch sequence is terminated by the onset of a period of uniform
34
magnetic polarity, 110 m. y. B, P, might be a more reasonable date
because this quiet magnetic zone can be interpreted as the Upper
Cretaceous Mercanton Interval of normal polarity (McElhinny and
Burek, 1971) which began around 110 m, y. B. P.
Recently, Larson and Pitman (1972) presented a global synthesis
of the Mesozoic geomagnetic time scale based on lineations in the
North Pacific. They correlate with good reason the Keathley sequence
to the Hawaiian lineations, but suggest that the Keathley sequence
occupies the interval from 155 to 110 m, y. B. P., and they question
the validity of the Lynch anomalies as polarity reversals, Instead,
they suggest that the Lynch anomalies result from bathymetric trends
and/or changes in the paleofield intensity, However, the Lynch
survey was made parallel to bathymetric and presumably structural
trends to avoid crossing fracture zones, The only track which does
cross structural trends is that part of profile E (Figure 7) which is
over the quiet zone, Following Ladd (1973), Larson and Pitman (1972)
conclude that spreading in the South Atlantic began during the Upper
Cretaceous quiet period, around 110 m, y. B, P. However, radiometric data all point to 140 to 130 m, y. B. P. as the age of opening
and the biostratigraphy of African coastal basins also suggests an
opening date earlier than 110 m. y. B. P. Basal units within the
Gabon, Congo and Cuanza basins are all pre-Aptian ( >110 m. y,)
(Belmonte and others, 1965), In the Gabon and Congo basins, the
35
lower sediments are Neocomian (>120 m. y. ) and perhaps older.
These facts conflict with Larson and Pitmants timing of the Hawaiian
and Keathley sequences, which requires that the Keathley sequence
end at 110 m. y. B. P. The reasonable correlation between the
Benguela and Lynch anomalies further supports the validity of the
Lynch anomalies and makes Vogt and others (1971) and Vogt and
JohnsonTs
(1971) date for the boundary between the Keathley and
Lynch sequences more reasonable,
It seems unlikely that the Lynch
anomalies lie over crust generated during a period of uniform
magnetic polarity, but the discrepancy between Larson and Pitman's
results and the existence of the Benguela and Lynch anomalies will
remain until further work can resolve it.
van Andel and Moore (1970) and Mascie and Phillips (1972a)
have described abyssal magnetic quiet zones in the South Atlantic
and the latter concluded that they are the result of seafloor spreading
during the Upper Cretaceous interval of normal polarity,
Unfortun-
ately, the boundaries of the zones are inadequately known, Mascle
and Phillips' chart (1972, Figure 5) suggests that the smooth zones
are continuous but non-linear areas, with sharp kinks in the two
African sequences, Such sharp changes in trend might be caused
by lateral offsets resulting fromfracture zones, Mascle and
Phillips (1972a)used data from PROJECT MAGNET flights (Mascle
and Phillips, 197'2b.. Figure 8 shows a preliminary reinterpretation
FIGURE 8: Location of Angola Basin and Cape Basin magnetic quiet
zone boundaries along PROJECT MAGNET flight lines
(Mascie and Phillips, 1972a) and CIRCE 9 ship track.
Lineation;of anomaly 33 indicated by light stippled line.
Identification of anomalies 31 and 32 provided for
reference to Heritzler sequence. Postulated fracture
zone offsets shown by heavy dashed lines. Tick marks
along PROJECT MAGNET lines indicate changes in
course. Vertical scale of CIRCE 9 is approximately
one-half that of the PROJECT MAGNET profiles shown
on figure. Bathymetry from Uchupi (1971). SM indicates
seamount anomaly.
37
50
Q0
50
00
50
50
1000
SM 33
31
CIRCE 9
LUAND
100
F
5°
20°
.ALVIS BAY
250
\\
LUDRITZ
I..
\
300
\
350
4Q0
A1
OWN
of the same data for the eastern South Atlantic plus the CIRCE 9
profile from van Andel and Heath (1970), with probable fracture zone
offsets of the quiet zones,
The most significant feature is a right
lateral offset across the Walvis Ridge, suggesting that fracture zones
border the more east-west trending segments of the ridge.
The quiet zone segment best defined by the PROJECT MAGNET
lines lies between 13°S and the Walvis Ridge, just west of the
Benguela anomalies, The western edge of this segment is marked
by a pronounced negative anomaly which strikes N12°W and is
slightly offset in a left-lateral sense, The eastern edge of the quiet
zone diverges both northward near 13°S and southward towards the
Walvis Ridge.
The reason is not understood but may be related to
a change in direction of spreading that took place during the transition
from the Benguela lineations to the quiet zone, North of
100S, the
quiet zone along the CIRCE 9 profile is much wider than anywhere
else. PROJECT MAGNET data in this region are inadequate,
however, to confirm this abrupt widening,
Irving and Couillard (1973), compiling continental magnetic
reversal data from 165 to 65 m. y. B, P., concluded that the best
compromise between land and marine data sets limits of 110 and
83 m. y. B, P. for the Cretaceous normal polarity interval, However,
there remains some uncertainty about the younger age limit of the
magnetic quiet zone. The 83 m, y. limit corresponds to the last
39
positive magnetic anomaly (33) of the Heirtzler sequence. Anomaly
33 (Figure 8: Mascie and Phillips, 1972a, Figure 4) is separated
from the quiet zoneby a smooth section of the magnetic profiles
which is shorter than the quiet zone. This short smooth section is
distinguished from the quiet zone mapped by Mascie and Phillips
(1972a) by the existence of a sharp negative anomaly along the
western boundary of the quiet zone, This short smooth interval,
separated from the quiet period, is recognizable not only in the
South Atlantic, but also in the North Atlantic (Vogt and others, 1971,
Figure 3), and Pacific (Raff, 1966, Figure 2), The sharp negative
anomaly bordering the younger edge of quiet zones in the Atlantic
and Pacific probably represents a reversal near 90 m. y. B. P.
observed by Russian investigators (Pecherski and Khramov, 1973),
and pointed out by Creer (1971), Thus, the quiet zone most likely
ends at this 90 m, y. reversal and anomaly 33 lies near 83 m, y. B. P,
The change in strike between the Benguela anomalies and the
western edge of the quiet zone represents a marked change in
spreading direction. Reorientation appears to have begun near the
end of anomaly lineation VII of the Benguela sequence.
If we assume
that the spreading rate was constant in the North Atlantic while the
Lynch anomalies were forming, it is possible to date the position
in the Lynch sequence which corresponds to the end of B-Vu time
and the change in spreading direction as approximately 120 m, y. B. P.
40
By 90 m. y. B. P., the reorientation to a new pole of rotation was
complete.
LePichon (1968) Showed that the South Atlantic must have opened
in at least two episodes of spreading defined by two poles of rotation
in order to reconcile the fit of South America and Africa (Bullard
and others, 1965) with the present rotation. The change in strike
between the Benguela lineations and the Angola Basin quiet zone
requires a shift in the pole of rotation between 140 and 90 m, y. B. P.
The earliest pole is defined by the strikes of the fracture zones along
the African margin.
The second pole is the same as that used by
Francheteau and LePichon (1972) for their initial opening (140 to
80 m. y. B. P.), which was determined from the strikes of projected
bathymetric trends of the equatorial South Atlantic. Small circles
around this second pole fit the northern scarps of the Walvis Ridge,
Rio Grande Rise and Falkiand Plateau, The two episodes of
spreading are shown in forward time sequence in Figure 9, beginning
from the fit of the 500 fathom contours of South America and Africa
of Bullard and others (1965). All rotations are made by moving the
South American plate and leaving Africa fixed. Table 1 summarizes
the rotations.
The angles of rotation were determined by rotating South
America away from Africa so that 1) the western edge of Benguela
anomaly belt VII ries along an approximately median position
41
FIGURE 9A 120 m y. B. P. reconstruction (end of B-Vu anomaly)
of South Atlantic obtained by rotation of South America
away from Africa (Table 1). Heavy solid lines are the
Libreville, Benguela, Luderitz and Cape Town lines
interpreted as fracture zones. Anomalies of Benguela
sequence on African plate where known from trackline
coverage on Figure 6. Odd numbered anomaly groups
are shown by stippled tone. Heavy dashed lines are
fracture zones through Benguela sequence. Stippled
line is 4000 meter contour of the margin and Rio Grande
Rise on South American plate. Light dashed line is
4000 meter contour of margin and Walvis Ridge on
African plate. Contours from Uchupi (1971).
FIGURE 9B 90 m. y. B. P. reconstruction (end of Upper Cretaceous
magnetic quiet period) of South Atlantic obtained by
second rotation of South America from Africa (Table 1).
Magnetic quiet zone (Mascie and Phillips, 1972a) shown
as horizontal lines on South American plate and dots on
African plate. All other symbols same as on Figure 9A.
/3FRlCA
SOUTH
AMERICA
:'%
,)
43
TABLE 1
Rotations used for Figures 9 and 13
Rotation
Pole
Plate
Lat.
Long.
Angle
44, 0
-30. 6
+57. 0
5. 0
-26, 0
-19. 5
90 m. y. B. P. reconstruction4
21, 5
-14.0
-12. 0
Equivalent to5
69. 0
-11. 0
+35. 0
Fit of Mid-Atlantic Ridge to 6
90 m,. y. B. P. reconstruction
69. 0
-11, 0
-17.5
South America
Fit of 500 fm contours2
120 m, y. B, P. reconstruction3
Not rotated
Africa
'Positive rotation of plate is in counter-clockwise sense
looking down at pole.
2Bullard and otherst (1965) fit of South America and Africa.
3End of B-Vu time, Figure 9A.
4End of Cretaceous normal polarity period, Figure 9B.
5Total post-90 m. y. rotation.
6Position of Mid-Atlantic Ridge crest on Figure 13. This
rotation follows rotation about pole at 21. 5°N, 14. 0°W;
Mid-Atlantic Ridge is fixed to South America plate during
rotation.
44
(Figure 9A) and 2) the seaward boundaries of the African and South
American quiet zones coincide (Figure 9B). The boundaries of the
magnetic quiet zones of Figure 9B are those mapped by Mascle and
Phillips (1972a).
These rotations do not necessarily describe the pattern of
movement, but only the relative motions required to fit the isochronous boundaries on each plate separated by an active ridge.
Because
of lack of data the reconstructions are only approximations to
successive configurations of the early South Atlantic, There are no
data off South America to establish the existence of a set of anomalies
comparable to the Benguela sequence, and symmetric spreading
during this period has been assumed. The second rotation permits
a test of fit between two magnetic boundaries across the spreading
ridge, Mascle and Phillips note that the quiet zone south of the Rio
Grande Rise is approximately 15% wider than its complement south
of the Walvis Ridge when measured along small circles drawn about
the rotational pole of Bullard and others (1965). The reconstruction
of Figure 9B indicates that the greater width of the Argentine Basin
zone follows from its increased distance from the pole at 21. 5°N,
14. O°W.
The remaining rotation required to place South America
in its present position is -35. 0 about 69. 0°N, 11. 0°W. This pole
disagrees with the present poles of rotation (62°N, 36°W or 58°N,
37°W) for the South Atlantic determined by Morgan (1968) and
45
LePichon (1968). Since this is an average pole for the entire
Cenozoic and Late Cretaceous, however, this difference would not
seem to be significant.
46
RELIEF AND STRUCTURE OF THE WALVIS RIDGE
Data for the structural study of the Walvis Ridge come from
continuous depth and seismic reflection recordings of the R. V.
Chain 99/4 and CIRCE 8 cruises. The area of study covered by these
cruises lies within 4°E to 11°E and 18°S to 27°S (Figure 10) and
contains six Chain 99/4 traverses across the Walvis Ridge north of
25°S and a star- shaped CIRCE 8 survey within the southern portion
of the study area that includes six lines of seismic reflection data.
Another CIRCE 8 line crosses the Walvis Ridge where it joins the
continental margin. This material and data of the University of Cape
Town (Simpson and Forder, unpublished map, 1967) were used for the
bathymetric base map of Figure 10. The bathymetry of the inner
margin facing southeast and northeast towards Africa has been
considerably improved by the addition of the Chain 99/4 and CIRCE 8
trackline s.
The seismic reflection data within the study area provide
coverage that is rather widely spaced yet detailed enough to yield a
tentative structural map of the upper section of oceanic crust.
Profiles representative of the structural character of the Ridge are
shown in Figure 11.
Based on the general outline of the Walvis Ridge as well as
dominant topographic trends of its uplands, the Walvis Ridge can be
47
FIGURE 10 Basement structure of the Walvis Ridge east of 4°E
superimposed on bathymetry. Structural interpretation
based on seismic reflection data from ship tracks of
the Chain 99/4 and CIRCE 8 cruises which are shown as
light stippled or solid lines. Seismic profiles in Figure
12 were recorded along the lettered solid tracklines and
selected to be representative of the structural character
of the Walvis Ridge. Bathymetric base map contoured
from data of Simpson and Forder (1967) with the addition
of depth profiles taken along each of the Chain 99/4 and
CIRCE 8 ship tracks. Contours are in uncorrected
meters.
48
180
2O
2 2G
-
EWING
$EMOUNT
24°
OB$E
°
'
_--
)
BASEMENT TROUGH
00
-
BASEMENT RIDGE
26°
28°
60
8°
10°
12°
1t
subdivided into three segments. Between the continental margin and
23°S, the relief trends nearly N60°E. From 23°S to 25. 5°S the
dominant topographic trend is N25°W although minor relief associated
with faulting strikes
N600E.
South of 25. 5°S the dominant trend
changes again to approximately N60°E. These regions are referred
to respectively as the northern, central and southern segments. The
central segment, with a width of about 300 kilometers, is the broadest part of the Walvis Ridge which narrows to under 170 kilometers
in the northern segment, Small ridges with a surface relief of 500
to 1500 meters occur over the Ridge uplands. They are linear and
in most places parallel the major bathymetric trends. The inner
margin of the Walvis Ridge consists of topographic scarps which are
approximately perpendicular to each other and give it a blocky
appearance. The outer margin of the Ridge, has a more broadly
curved outline between 8°E, 20°S and 5°E, 25, 5°S, but this contrast
with the inner margin may be more apparent than real because of
inadequate bathymetric control, To the northeast and southwest of
this wide arc, the trends are more nearly parallel to those of the
inner margin.
Reflection profiles (Figure 11) show that the acoustic basement
consists of discontinuous, hummocky reflectors indicating that the
basement is volcanic, The smooth appearance of the acoustic
basement in the central part of the ridge may be due either to a hard
50
FIGURE 11 Line drawings of seismic reflection profiles across
Walvis Ridge. Locations shown in Figure 11. Profiles
AB, BC, DE from Chain 99/4; vertical exaggeration
approximately 8. Profiles FG and HI from CIRCE 8;
vertical exaggeration approximately 14. Vertical scales
in two-way reflection time. Horizontal scales approximate. Acoustic basement shown with thicker line than
overlying reflectors. Fault interpretations indicated by
heavy dotted lines.
cQ
[I
I?
Ic,
/1/
I 1/
[0
Zo
_jf
0[
Ii
0
Sc_
BENGUELA
I
I
Fz,
BLLA
V/S WAL
W4LV/S
-
NORTH
NORTH
If
C)
0
SEC
0
-
r------'------------i---m
-
SEC
Il
0000-0
I-
I
I
SEC
I
SEC.
I
0
52
sediment layer such as chert, or to the presence of smoothly layered
extrusives, since Deep Sea Drilling data indicate that extrusive
basaltic basement can be quite smooth (Ewing and others, 1972).
Faults on the reflection profiles commonly affect only the basement, but some faulting extends upwards into the sedimentary
reflectors, Sedimentation appears to have been largely controlled
by deposition within structural troughs and by erosion or non-
deposition over the more exposed basement ridges and flat regions
of basement of the central segment, Even with this smoothing effect
of sedimentation, the Walvis Ridge topography strongly reflects
the underlying system of basement faulting. Figure 10 shows the
interpretation of basement structure derived from reflection data
and topography.
The structural framework of the upper oceanic crust here is
controlled by three sets of faults. The first set is associated with
the Benguela Fracture Zone and strikes generally N25°E, The
remaining two sets strike approximately N60°E and N25°W.
These
latter faults control the topographic expression of the Walvis Ridge.
Along the northern boundary of the northern segment of the Ridge,
the N600E set of faults disrupts the trend of the Benguela Fracture
Zone.
The following discussion considers these three sets of faults
and associated structural features in more detail.
The Benguela Fracture Zone is defined by the seismic
53
reflection data as approximately 135 kilometers wide in the region
where the Walvis Ridge joins the continental margin. Within this
zone, parallel and sub-parallel faulting is associated with basement
ridges and troughs. The faults are completely buried by sediments
and, on Figure 10 with its 500 m contour interval, have little or no
surface topographic expression. The faults can be recognized by
vertical offsets of the basement,
The major offset denoted as the
Benguela Fracture Zone on profiles AB and BC strikes N25 °E. All
faults on these two profiles, except those forming the outer northern
boundary scarp, are part of the Benguela set, The Benguela
structures appear to end at the northern boundary of the Walvis Ridge.
Along this northern boundary two prominent basement ridges are
uplifted along scarps which strike N65°E (Figure 10, Figure 11,
profiles AB, BD). However, south of this boundary, the basement
ridges trend approximately S25°W along faults of the Benguela set,
Between these two basement ridges, a small topographic indentation
of the Ridge boundary occurs near 9. 5°E, 19. 3°S along an intersection of N25°E Benguela faults and N60°E boundary faults.
South of 22°S, the identification of the Benguela Fracture Zone
is uncertain. Along the flank of the inner margin of the southern
segment, near 26°S, 7-8°W, of the Walvis Ridge, the edge of a
wide fault terrace strikes nearly N25°E. The faults along which
this terrace is uplifted are shown in profile HL They may be related
54
to the Benguela Fracture Zone, but they do not extend northward
through the Cape Basin across profile DE. However, the location and
alignment of the Ewing Seamount and a smaller one north of it
suggest that their formation was controlled by the Benguela Fracture
Zone.
The discussion now focuses on the two sets of faults which
strike N60°E and N25°W and dominate the structural framework of
the Walvis Ridge.
The Ridge itself appears to consist of a series of
crustal blocks offset right-laterally along the N600E set. The large
right-lateral offset of the nagnetic quiet zones to the north and south
of the Walvis Ridge (Figure 8) indicates that this faulting is related
to transform shearing. The most prominent examples of the N600E
set on Figure 11 are the North Walvis Fracture Zone of profiles AB
and BC, and the exposed high basement scarp approximately 70
kIlometers from the eastern end of profile FG. Within the central
segment of the Ridge, the structural features on profile DE between
22°S and 23°S lie along extensions of the nearby boundary scarps.
In the southern segment, a high basement ridge and trough near
5. 5°E, 26. 25°S run along the topographic axis but are connected
to the N60°E boundary scarp which occurs between 25°S and 26°S.
Where seismic data are lacking, the linear relief of the Walvis Ridge
uplands indicates that the major faults, which form SE-facing
boundary scarps along the inner margin, continue southwestward
55
across the Ridge. The two positive basement blocks centered on
profile FO have been uplifted along faults of the N25°W set, The
basement high to the west appears to be offset right-laterally at
5. 75°E, 25, 5°S across a fault extending from the N600E boundary
fault between 24, 5°S and 25°S, There are no seismic data over the
area in which a similarly offset eastern block should be found.
However, the observed eastern block appears to be terminated to
the north by down-faulting,
van Andel and Heath (1970) pointed out the existence of
numerous normal faults over the eastern flank of the Mid-Atlantic
Ridge, which parallel the ridge crest and are nearly orthogonal to
intervening fracture zone offsets. The NW-NE orthogonal structure
of the Walvis Ridge outside the region affected by the Benguela
structures, is strikingly similar to the inherited tectonic fabric of
the volcanic layer 2 within a laterally offset, spreading sea floor,
The perpendicular faults through the Walvis Ridge suggest a similar
origin for its structure,
56
TECTONIC EVOLUTION OF THE WALVIS RIDGE
I have argued above, using lineations of the Gondwanan zones
of weakness as well as an interpretation of magnetic lineations off
the Angolan and South West African continental margin, that the
initial opening of the South Atlantic occurred about a pole at 5°N,
26°W.
The magnetic data suggest that a significant reorientation
of spreading direction occurred somewhat before the onset of the
Upper Cretaceous interval of uniform magnetic polarity.
The initial spreading direction south of Luanda, Angola, was
oriented approximately S25°W along the long transform shear called
here the Benguela Fracture Zone, The spreading axis began to
reorient after formation of anomaly group B-Vu. A little earlier,
near the end of B-Vu time, the edges of the South America and
African continents had become completely separated. The
continental blocks must have remained together longest, along the
Benguela Fracture Zone since this was the largest initial offset
between the two continents. Once the two continents were no longer
juxtaposed, the rotational pole moved northward. This suggests that
location of the initial pole of rotation for the South Atlantic opening
was controlled by the pre-existing structure of Gondwana. Figure 12
shows the configuration of spreading centers and fracture zones at
approximately 120 m. y. B. P. in the South Atlantic, It is based on
57
FIGURE 12 Paleotectonic interpretation of early South Atlantic
opening (approximately 120 m. y, B. P.). Seafloor
spreading elements based on extrapolation of data
from Figure 9A. Ridge crests shown by double solid
and dashed lines, transform faults by single heavy
lines and short dashed lines, Inactive fracture zones
shown by heavy dashed lines and lighter dashed lines.
Oceanic crust depicted by diagonal lines. 400, 2000,
and 4000 meter contours of South American (stippled
lines) and African (solid lines) plates from Uchupi
(1971).
f:.
59
extrapolation of the data in Figure 6 and the rotation of Figure 9A.
The reconstruction of Figure 12 shows that the tectonics of the early
opening phase of the South Atlantic were dominated by long linear
shear elements separated by short rifting segments. Lachenbruch
and Thompson (1972) have shown that resistance to shearing may be
less than resistance offered to rifting; hence the configuration for
the initial spreading phase is quite plausible. Studies of the tectonics
of the Gulf of California and the Gulf of Aden, both geologically very
young rifted basins, show a very similar pattern of initial separation
of continental crust, predominatly along en echelon shears separated
by shorter spreading ridge segments.
Figure 13 shows the amount of crust generated by 90 m. y. B. P.,
beyond the 120 rn. y. B. P. plate boundaries. The approximate posi-
tion of the accreting African plate margin at 90 m. y. B. P. is
indicated by the seaward boundary of the Angola and Cape Basin
magnetic quiet zones. The position of the spreading center across
the latitudes of most of the Walvis Ridge is unknown,
The evolution-
ary relationship of the Walvis Ridge and Rio Grande Rise is shown
by overlapping bathymetry of the South American and African plates.
The fit of the Walvis Ridge between the EW trending Columbia-
Trindade seamount chain along 20°S off Brazil and the Rio Grande
Rise to the south is striking. The Columbia-Trindade seamount
chain is nearly aligned with the northern boundary of the Walvis
FIGURE 13 Opening of South Atlantic at the end of Upper Cretaceous
period of uniform polarity (approximately 90 m. y. B. P.).
Position of 90 m. y. plate boundary estimafd by seaward
boundaries of the Angola and Cape Basin magnetic
quiet zone; shown by heavy solid and dashed lines. Grey
stippled tone within double solid lines depicts present
Mid-Atlantic Ridge crest rotated back to 90 m. y. position (Table 1). Configuration of present ridge crest
based on positions of earthquake epicenters (Stover,
1968) and Ladd and others (1973) South Atlantic chart of
magnetic anomalies. Pre-120 m. y. oceanic crust from
Figure 12. Inset shows 120 m. y. boundaries and small
circles about pole at 21. 5°N, 14°W.
/
/
/
/
/
/
/
/
/
/
/.
/
/
/
/
/
N
62
Ridge along a small circle of the rotational pole (21. 5°N, 14°W).
The northern scarp of the Rio Grande Rise is likewise aligned with
the furthest westernprojection of the Walvis Ridge shown, beyond
the point where it splits into two narrow ridge segments. Figure 13
also includes the present configuration of the Mid-Atlantic Ridge
crest which has been rotated using the poles and angles shown in
Table 1. North of the Walvis Ridge and south of the Rio Grande Rise
the present rotated ridge crest is centered near the 90 m. y. B. P.
plate boundary. Deviations of the present ridge crest from this
plate margin suggests asymmetric spreading or an eastward jump
of the spreading centers after 90 m,
y,
These deviations are
discussed in more detail later.
Interpretation of the Walvis Ridge structure suggests that it
consists of crustal blocks offset right-laterally along N60°E
trending fracture zones which extend through the Ridge and border
it to the north and south, The high NW and SE-facing boundary
scarps as well as the N60°E striking basement ridges and troughs
over the Walvis Ridge uplands may be explained as secondary tectonic
features related to vertical movements along en echelon fracture
zones. Vertical movements play a significant role in the dynamics
of fracture zones (van Andel, 1971), possibly resulting from low
density roots of altered mantle material beneath them as suggested
by gravity and seismic refraction data for the Mendocino (Dehlinger
63
and others, 1970) and the Rivera Fracture Zones (Gumma, 1973).
Since the N600E faults generally disturb only the basement, uplift
of the Walvis Ridge must have occurred very early in its history.
The North Walvis Fracture Zone (Figure 11) along the northern
boundary of the Walvis Ridge strikes generally N65°E between 7°E
and fl°E, and cuts across the N25°E structures within the Benguela
Fracture Zone (Figure 10). This scarp is relatively straight except
between 9°E and 10, 5°E where its linearity is interrupted by
crossing the Benguela zone, suggesting that the Benguela structures
formed prior to the N60°E trending faults. Formation of the
northern boundary fault then must have required breaking through
the older Benguela Fracture Zone, According to Menard and
Atwater (1969), a reorientation of the sense postulated here should
result in a leaky fracture zone with slow transverse spreading, not
in a break through older crust, However, the Ascension Fracture
Zone shows a similar anomalous break through older crust
(van Andel and others, 1973),
Projection of the boundaries of the Benguela Fracture Zone
from the structures between the Walvis Ridge and continental margin
to the southwest, shows that the orthogonal boundary scarps of the
inner Walvis Ridge margin lie within its breadth (Figure 14). The
boundary scarps may thus mark the configuration of the spreading
and transform offsets which formed as tension began to rift apart
FIGURE 14
Postulated reorientation of short spreading centers
within Benguela Fracture Zone during the 120 m. y.
spreading reorientation. Benguela Fracture Zone
boundaries estimated from width of faulted zone
trending N25°E near junction of Walvis Ridge and
continental margin, and projected to the southwest.
N25°W trending boundary scarps along inner margin
of Walvis Ridge interpreted as initial spreading
centers. 2000 and 4000 meter contours of Walvis
Ridge included for reference, The inset diagram
shows reorientation of stress components acting on
the Benguela Fracture Zone at the earth's surface.
65
/\J1
I
I
/
(I
\\\
/
(/
/
/ ----
/
200
/
000
//ZA/////_
220
4/
240
/
,1
///
26°
-
/
28°
-
I
4°
6°
//
POSTULATED REORiENTATONOF
STRESS COMPONENTS ALONG
ENGUELA FRACTURE ZONE
I
8°
I
0°
I
12°
the Benguela Fracture Zone during the reorientation. The difference
in spreading directions between the Benguela and Walvis Ridge
Fracture Zones is 40 degrees. This dramatic reorientation may
have precluded a leaky fracture zone from developing along the
Benguela Fracture Zone and instead permitted short spreading
centers to develop almost immediately.
The model for the tectonic evolution of the Walvis Ridge
(Figure 15) hypothesizes asymmetric spreading of the short
spreading segments to account for the eastward deviation of the
present rotated Mid-Atlantic Ridge crest with respect to the 120
m. y. plate boundaries.
The asymmetric spreading is postulated
here only for the crust on which the Walvis Ridge is located.
Whether the continued eastward deviation of the present ridge crest
is related to asymmetric spreading over the Rio Grande Rise or to a
jumping ridge crest is not known. The hypothesis of asymmetric
spreading over the Walvis Ridge is supported by the continuing
proximity of the spreading axes to their initial loci along the
Benguela Fracture Zone. The extreme width and length of the offset
of the Benguela Fracture Zone may have created a stationary zone
of weakness within the lithosphere along which spreading was
preferred. Thus the spreading rate for the African plate may have
been very slow or close to zero, allowing the spreading axes to
remain for some time near this postulated zone of weakness, The
67
FIGURE 15
Proposed model for tectonic evolution of the Walvis
Ridge, Short dashed line shows 120 m. y. plate
boundaries separated by 70 opening about pole at
21. 5°N, 14 0°W, Heavy solid lines show position
of plate boundary after spreading reorientation at
120 m. y. B. P. To the north and south of the Walvis
Ridge the new spreading centers have jumped to the
west of the original 120 m. y. plate boundary breaking
off triangular and jagged crustal blocks which are now
part of the African plate. Spreading reorientation
across the present Walvis Ridge occurred within the
Benguela Fracture Zone (Figure 14). Fractured zones
fall along small circles of the rotational pole and
coincide with the set of N60°E faults across the Walvis
Ridge. Asymmetric spreading postulated for crust
over the Walvis Ridge; elsewhere the model assumes
symmetric spreading, although this may not be true
for crust over Rio Grande Rise. 400, 2000 and 4000
meter contours shown by stippled lines on South
American plate and solid lines on African Plate,
Bathymetry of Walvis Ridge and Rio Grande Rise
continued across plate boundary. Bathymetry from
Uchupi (1971) except Walvis Ridge contours taken
from Figure 10.
0
0
a
0
0
0
\
0
0
68
combination of asymmetric spreading ridge segments and closely
spaced transform faults at the margin of this older fracture zone
may have permitted deep alteration of underlying mantle, and the
formation of a broad low density root. If the elevation of the Walvis
Ridge occurred very early in its history as suggested above, then
the Ridge must presently be in isostatic compensation.
North and south of the Walvis Ridge, the 90 m. y. plate
boundary (Figure 13) is off center to the west with respect to the
120 m. y. boundaries. The linear EW trends of the Columbia-
Trindade seamount chain and the continental rise south of the Sao
Paulo Plateau both lie within the pre-120 m. y. South American
plate in which spreading was oriented much more NE-SW. These
observations suggest that during reorientation the spreading center
jumped to the west and broke off triangular and jagged blocks of
older crust which must now be part of the African plate north and
south of the Walvis Ridge (Figure 15). If this is true, then the 90
m. y. plate margin is centered between the older adjusted ridge
crests. The trend and location of the 90 m. y. boundary on the
African plate north of the Walvis Ridge on Figure 16 conflicts
slightly with the magnetic anomaly lineation B-IX in Figure 6. Since
correlation of the B-IX anomalies between the CIRCE 8 and Chain
99/4 (Figure 5) is poor, the trend of the adjusted plate margin here
is probably more like that shown in Figure 15. The postulated
I'.]
jumps of the spreading centers as well as the formation of short
spreading segments within the Benguela Fracture Zone during the
first change in spreading direction suggest an instantaneous
reorientation of the spreading axes as well as the transform faults.
Additional work is needed to clarify the tectonic relationship
between the Walvis Ridge and the Rio Grande Rise. The tectonic
model shows that a large section of the Walvis Ridge had been
generated before most of the Rio Grande Rise began to form. The
evolution proposed here indicates also a more southern path of drift
for the Rio Grande Rise than has been previously proposed
(LePichon and Hayes, 1971; Francheteau and LePichon, 1972).
Reconstruction of the early South Atlantic (135-120 m. y. B. P.
spreading direction proposed in this paper differs from the model
suggested by Francheteau and LePichon (1972). According to them,
the trends of equatorial Atlantic fracture zones extrapolated into
the West African margin along Ghana and the Ivory Coast, can be
fitted to small circles about a pole at 21. 5°N, 14, 0°W. Rotation
about this pole would have occurred between 140 m, y. B. P. and
80 m. y. B. P. According to this paper the initial spreading of the
South Atlantic occurred about a pole at 5°N, 26°W between 135 m. y.
and 120 m. y. B. P.
This apparent conflict between the early
spreading directions in the equatorial South Atlantic and the area
further south described in this paper, may have arisen from jumps
71
of the Mid-Atlantic Ridge spreading centers during the major
reorientation at 120 m. y. B. P. Rotation of the present zone of
epicenters along the Mid-Atlantic Ridge crest (McKenzie and Sclater,
1971, Figure 40) back to the Bullard fit of South America against
Africa (Bullard and others, 1965) shows deviations from the initial
rift along most of its length. Present epicenters lying between the
equatorial fracture zones and the Walvis Ridge are placed within
South America between 13°S and 20°S (with respect to South America)
by this rotation. The tectonic hypothesis of Figure 15 suggests a
westward spreading jump in these latitudes during the 120 m. y.
reorientation and breaks through older crust of the South American
plate by the spreading centers. Thus the positions of present rotated
epicenters deviate to the west of the initial rift between 13° and 20°S.
A similar rotation of the present equatorial epicenters (McKenzie
and Sclater, 1971) places them within Africa near 10°W (with
respect to Africa), particularly where the Romanche and Chain
fracture zones are projected into the continent by Francheteau and
LePichon (1972).
The eastward deviation of the epicenters from the
initial rift near the Romanche and Chain Fracture Zone projections,
suggests that during the 120 n-i. y. reorientation, the spreading
centers may have jumped to the east, and that these fractures broke
through the older sea floor lineations produced by opening parallel
to the Pan-African zones of weakness. This is supported by the
72
abrupt change in direction at the eastern end of the Romanche
Fracture Zone trend where it parallels the continental margin along
a steep escarpment called the Ivory Coast-Ghana Ridge (Arens, and
others, 1971). The Ivory Coast-Ghana Ridge emerges from the
continental margin near Accra where the Buem-Togo Thrust zone
intersects the margin (Grant, 1969). This thrust zone is of PanAfrican age and generally parallels the Pan-African offsets of the
continental margin to the south. The eastern terminus of the Ivory
Coast-Ghana Ridge has the same strike as that of the onshore
Buem-Togo Thrust Zone and may be a structural remnant of the
initial opening between South America and Africa whose direction of
spreading was controlled by the Pan-African structures.
73
BIBLIOGRAPHY
Amaral, G., U. Cordani, K. Kawashita, and J, Reynolds. 1966.
K-Ar dates from basaltic rocks of southern Brazil Geochim.
Cosmochim, Acta, 30:159,
Arens, G,, J. R, Delteil, P. Valery, B. Damotte, L. Montadert,
and P. Patriat. 1971. The continental margin of Ivory Coast
and Ghana.
In: Geology of the East Atlantic continental
margin. ICSU/SCOR Working Party 31 Symposium,
Cambridge, 1971. F.M. Delaney, Ed. , Inst. Geol. Sci,
Report No, 70/16.
Baumgartner, T. R. and Tj, H, van Andel, 1971. Diapirs of the
continental margin of Angola, Africa. Geol. Soc. Am, Bull.
82:793- 802,
Belmonte, Y., P. Hirtz, and R. Wenger, 1965. The salt basins of
the Gabon and the Congo (Brazzaville): A tentative paleogeographic interpretation. In: Salt Basins Around Africa,
W. Q. Kennedy, Ed. , Inst. Petroleum, London,
Bryan, G. M. and E, S. W. Simpson. 1971, Seismic refraction
measurements on the continental shelf between the Orange
River and Cape Town, In: The Geology of the East Atlantic
Continental Margin, ICSU/SCOR Working Party 31 Symposium,
Cambridge 1971. F.M. Delaney, Ed, Inst. Geol, Sci.
Report No. 70/16. pp 187-198.
Bullard, E, C.,, J, E. Everett, and A. G. Smith, 1965, The fit of
the continents around the Atlantic. Phil. Trans. Roy. Soc.
London, A-258:41-51.
Burk, C.A. 1968. Buried ridges within continental margins.
Trans. N. Y. Acad. Sci, , 3 0:397-409,
Burke, K. 1969, Seismic areas of the Guinea coast where Atlantic
fracture zones reach Africa, Nature, 222:655-657.
Burke, K., T. F. J. Dessauvagie, and A. .1. Whiteman. 1971.
Opening of the Gulf of Guinea and geological history of the
Benue Depression and Niger Delta. Nature P.S., 233:51-55,
Cain, J. C., S. Hendricks, W, E, Daniels, and D. C. Jensen. 1968.
74
Computation of the main geomagnetic field from spherical
harmonic expansion. National Space Flight Data Center Pub.
No. 68-11.
Clifford, T. N. 1968. Radiometric dati.ng and the pre-Silurian
geology of Africa. In: Radiometric Dating for Geologists,
Interscience.
E. I. Hamilton and R. M. Farquar, Eds.
Clifford, T. N. 1970. The structural framework of Africa. In:
African Magmatism and Tectonics. T. N. Clifford and
I. G. Gass, Eds, Oliver and Boyd, Edinburgh.
Cordani, U. G.,, G. C. Melcher, and F. F. M. de Almeida. 1968.
Outline of the Precambrian geochronology of South America.
Can, J, Earth Sci. , 5:629-632,
Creer, K. M. 1971. Mesozoic paleomagnetic reversal column.
Nature, 233:545-546,
Dehlinger, P., R. W. Couch, D. A. McManus, and M. Gemperle.
1970. Northeast Pacific structure, In: The Sea, Vol. IV,
Pt. 2, A. E. Maxwell, Ed.
Dickson, G. 0.,, W. C. Pitman and J. R. Heirtzler. 1968. Magnetic
anomalies in the South Atlantic and ocean floor spreading.
J. Geophys. Res. , 73:2087-2100.
Dietz, R. S. 1961, Continental and ocean basin evolution by
spreading of the sea floor, Nature, 190:854-857.
Dietz, R. S. and J. C. Holden. 1970. Reconstruction of Pangea:
Breakup and dispersion of continents, Permian to present.
J. Geophys. Res, , 75:4939-4956.
Dingle, R. V. 1973. The geology of the continental shelf between
Luderitz and Cape Town (South-West Africa), with special
reference to Tertiary strata. (in press),
Drake, C. L., M. Ewing, and G. H. Sutton. 1959. Continental
margins and geosynclines: The east coast of North America
north of Cape Hatteras. In: Physics and Chemistry of the
Earth, Vol. 3, Pergammoii, New York.
Emery, K. 0., E. Uchupi, J. D. Phillips, C. 0. Bowin, E. T. Bunce,
and S. T. Knott. 1970. Continental rise off eastern North
America. American Assoc. Petrol. Geol. Bull., 54:44-108.
75
Ewing, J. I. and C. H. Hollister. 1972. Regional aspects of Deep
Sea Drilling in the western North Atlantic. In: Hollister,
C. D., J. I. Ewing and others, 1972, Initial Reports of the Deep
Sea Drilling Project, Vol. XI, Washington, D. C. , U. S. Govt.
Printing Office,
Ewing, M., W. J. Ludwig, and J. I. Ewing. 1963. Geophysical
investigations in the submerged Argentina coastal plain, 1,
Buenos Aires to Peninsula Vodez. Geol. Soc. Am. Bull.,
74:275-292.
Francheteau, J. and X, LePichon, 1972. Marginal fracture zones
as structural framework of continental margins in South
Atlantic Ocean, Amer. Assoc, Petrol, GeoL Bull,
56:275-292.
Grant, N. K. 1969. The Late Precambrian to Early Paleozoic
Pan-African orogeny in Ghana, Togo, Dahomey, and Nigeria.
Geol. Soc. Am. Bull., 80:45-56.
Gumma, W, H.
1973. An interpretation of the gravity and magnetic
anomalies of the Riviera Fracture Zone, eastern Pacific
Ocean. M. S. Thesis, Oregon State University, Corvallis,
Oregon.
Heirtzler, JR., G.O. Dickson, E.J. Herron, W.C. Pitman, and
X. LePichon. 1968. Marine magnetic anomalies, geomagnetic
field reversals, and motions of the ocean floor and continents.
J. Geophys. Res,, 73:2119-2136,
Hess, H. H. 1962, History of ocean basins. In: Petrologic Studies:
A volume to Honor A. F. Buddington, A. E. J. Engel,
H. L. James, and B. F. Leonard, Eds, Geol, Soc. Am.
New York.
Hurley, P. M. and J. R. Rand, 1969. Pre-drift continental nuclei.
Science, 164:1229-1242.
Irving, E and R. W. Couillard. 1973. Cretaceous normal polarity
interval. Nature P.S., 244:10-11.
Kennedy, W. Q.
The influence of basement structure on the
evolution of the coastal (Mesozoic and Tertiary) basins of
Africa, In: Salt Basins Around Africa, W, Q. Kennedy, Ed.,
Inst. Petroleum, London.
1965,
76
Lachenbruch, A.H. and G.A. Thompson. 1972. Oceanic ridges
and transform faults: Their intersection angles and
resistance to plate motion, Earth Planet. Sci. Lett.
15:116-122.
Ladd, J. W,,, G. 0. Dickson, and W. C. Pitman, 1973. The age of
the South Atlantic, In: The Ocean Basins and Margins: The
South Atlantic, A. E. M. Nairn and F. G, Stehli, Eds.
Plenum Pub, Corp., New York, (in press).
LePichon, X. 1968. Sea-floor spreading and continental drift.
J. Geophys. Res., 73:3611-3697.
LePichon, X. and D. E, Hayes. 1971. Marginal offsets, fracture
zones, and the early opening of the South Atlantic, J. Geophys.
Res, , 76:6283-6293.
Leyden, R., G. Bryan and M. Ewing, 1972. Geophysical reconnaissance on African shelf: 2, Margin sediments from Gulf
of Guinea to Walvis Ridge. American Assoc. Petrol. Geol.
Bull.
,
56:682-693.
Larochelle, A. 1969. Paleomagnetism of the Monte region Hills:
further new results, J. Geophys. Res, , 74:2570-2575.
Larson, R. L. and W. C. Pitman, 1972. World-wide correlation of
Mesozoic magnetic anomalies, and its implications. Geol.
Soc. Am, Bull,, 83:3645-3662,
Mascle, J, and J. D. Phillips,
197 2(a), Magnetic smooth zones in
the South Atlantic. Nature, 240:80-84.
Mascie, J, and J. D, Phillips.
197 2(b). Eastern Atlantic continental
margin geomagnetic data. Woods Hole Oceanographic Inst.
Ref. No. 72-3, unpublished manuscript,
Maxwell, A. E., R. P. Von Herzen, K. J. Hsu, J. E, Andrews,
T. Saito, S. F, Percival, Jr., E. D, Milow, and R. E. Boyce,
1970. Deep Sea Drilling in the South Atlantic,
168:1047- 1059.
Science,
McElhinny, M. W, and P. J. Burek. 1971. Mesozoic paleomagnetic
stratigraphy. Nature, 232:98- 102.
McElhinny, M. W.,, J. C, Briden, D, L. Jones and A. Brock, 1968.
Geological and geophysical implications of paleomagnetic
77
results from Africa. Rev. Geophys. , 6:201-238.
McKenzie, D. P. and R. L, Parker, 1967. The North Pacific:
an example of tectonics on a sphere. Nature, 216:1276-1280.
McKenzie, D. P. and 3. G. Sciater. 1971. Evolution of the Indian
Ocean since the Late Cretaceous. Royal Astron. Soc. Geophy.
J. , 25:437-528,
Menard, H. W. and T. Atwater. 1968. Changes in direction of sea
floor spreading. Nature, 219:463-467.
Morgan, W. 3. 1968, Rises, trenches, great faults and crustal
blocks, Jour. Geophys, Res,, 73:1959-1982,
Pautot, G., V. Renard, 3. Daniel, and 3. Dupont. 1973.
Morphology, limits, origin and age of the salt layer along
South Atlantic African margin. (in press).
Pechersky, D.M. and A, N. Khramov. 1973. Mesozoic Paleomagnetic scale of the U. S. S. R, Nature, 244:499-501.
Pitman, W. C. and M. Taiwani, 1972. Sea-floor spreading in the
North Atlantic. Geol, Soc. Am, Bull, , 83:619-646.
Rabinowitz, P. D. 1972, Gravity anomalies on the continental
margin of Angola, Africa. 3. Geophys. Res, , 77:6327-6347.
Rabinowitz, P. D. and M. Talwani. 1969. Gravity anomalies in the
western North Atlantic Ocean. Abstract, Geol. Soc. Am.
Annual Meeting, Atlantic City, N. 3,
Raff, A. D.
1966. Boundaries of an area of very long magnetic
anomalies in the northeast Pacific, J. Geophys, Res.
71:2631-2636.
Siedner, G. and 3. Miller. 1968, K-Ar determinations on basaltic
rocks from southwest Africa and their bearing on continental
drift, Earth Planet, Sci, Lett. , 4:451-458.
Simpson, E. S. W. 1971. The geology of the south-west African
continental margin. In: The Geology of the East Atlantic
Continental Margin. ICSU/SCOR Working Party 31
Symposium., Cambridge, 1971. F, M. Delaney, Ed., Inst.
Geol, Sci,, Report No, 70/16,
Simpson, E. S. W. and A. du Pies sis. 1968. Bathymetric, magnetic,
and gravity data from the continental margin of southwestern
Africa. Can. J. Earth Sci., 5:1119-1123.
Simpson, E.S.W. andE, Forder.
1967. SoutheastAtlantic and
Southwest Indian Oceans, unpublished bathymetry map,
University of Cape Town, Rondebosch, South Africa.
Stover, C, W, 1968. Seismicity of the South Atlantic Ocean.
J. Geophys, Res, , 73:3807-3820.
Talwani, M, and 0. Eldholm, 1972. Continental margin off
Norway: A geophysical study. Geol, Soc. Am. Bull.,
83:3575-36 06.
Uchupi, E. 1971. Bathymetric atlas of the Atlantic, Caribbean
and Gulf of Mexico, Woods Hole Oceanographic Inst. , Ref.
No. 71-72.
1969. International tectonic map of Africa,
1:5, 000, 000, Paris.
UNESCO-ASGA.
van Andel, T. H. 1971. Fracture zones.
Sciences: Geophysics, 1:159-166.
Comments on Earth
van Andel, T. H. and S. E. Calvert. 1971... Evolution of sediment
wedge, Walvis Shelf, southwest Africa. J. Geol.,
79:585-602.
van Andel, T. H. and G. R. Heath. 1970. Tectonics of the MidAtlantic Ridge, 6°-8 South latitude. Marine Geophys. Res.
1:5-36.
van Andel, T. H. and T. C. Moore, Jr. 1970. Magnetic anomalies
and seafloor spreading rates in the northern South Atlantic.
Nature, 226:328-330.
van Andel, T. H., D. K. Rea, R. P. Von Herzen and H. Hoskins.
1973. Ascension Fracture Zone, Ascension Island, and the
Mid-Atlantic Ridge. Geol. Soc. Am. Bull. , 84:1527-1546.
Vine, F. J. 1966. Spreading of the ocean floor: new evidence.
Science, 154:1405-1415.
Vine, F. J. and D. H. Matthews. 1963. Magnetic anomalies over
79
oceanic ridges. Nature, 199:947-949.
Vogt, P.R. and G. L. Johnson. 1971. Cretaceous seafloor
spreading in the western North Atlantic. Nature, 234:22-25.
Vogt, P.R., C.N Anderson, and D.R. Bracey.
1971.
Mesozoic
magne tic anomalies, sea-floor spreading, and geomagnetic
reversals in the southwestern North Atlantic. J. Geophys.
Res., 76:4796-4823.
Von Herzen, R. P., H. Hoskins, and T. H. van Andel. 1972.
Geophysical studies in the Angola diapir field. Geol. Soc.
Am. Bull., 83:1901-1910.
Wilson, J. T. 1965. A new class of faults and their bearing on
continental drift. Nature, 207:343.